To increase the availability in vehicle electrical systems that place high demands on a permanent energy supply, it is already known to use electrical system topologies that include an auxiliary battery. The state of the main battery is ascertained via an electronic battery management, in a relatively costly manner, and the auxiliary battery is switched in as a function of a constant threshold value. The battery current, the battery voltage and the battery temperature are normally used to determine the battery state.
From German Patent No. DE 198 45 562, a method for ascertaining the state of the electrical system of a vehicle is known that uses the measurement of the battery voltage. In that procedure, the battery voltage is averaged over a longer period of time. A critical state is recognized when the voltage drops below a defined voltage threshold.